1. Field of the Invention
The present invention generally relates to the art of microelectronic integrated circuits. In particular, the present invention relates to the art of computing delays for cells in ASICs.
2. Description of the Prior Art
An integrated circuit chip (hereafter referred to as an xe2x80x9cICxe2x80x9d or a xe2x80x9cchipxe2x80x9d) comprises cells and connections between the cells formed on a surface of a semiconductor substrate. The IC may include a large number of cells and require complex connections between the cells.
A cell is a group of one or more circuit elements such as transistors, capacitors, and other basic circuit elements grouped to perform a function. Each of the cells of an IC may have one or more pins, each of which, in turn, may be connected to one or more other pins of the IC by wires. The wires connecting the pins of the IC are also formed on the surface of the chip.
A net is a set of two or more pins which must be connected. Because a typical chip has thousands, tens of thousands, or hundreds of thousands of pins which must be connected in various combinations, the chip also includes definitions of thousands, tens of thousands, or hundreds of thousands of nets, or sets of pins. All the pins of a net must be connected. The number of the nets for a chip is typically in the same order as the order of the number of cells on that chip. Commonly, a majority of the nets include only two pins to be connected; however, many nets comprise three or more pins. Some nets may include hundreds of pins to be connected. A netlist is a list of nets for a chip.
Microelectronic integrated circuits consist of a large number of electronic components that are fabricated by layering several different materials on a silicon base or wafer. The design of an integrated circuit transforms a circuit description into a geometric description which is known as a layout. A layout consists of a set of planar geometric shapes in several layers.
The layout is then checked to ensure that it meets all of the design requirements. The result is a set of design files in a particular unambiguous representation known as an intermediate form that describes the layout. The design files are then converted into pattern generator files that are used to produce patterns called masks by an optical or electron beam pattern generator.
During fabrication, these masks are used to pattern a silicon wafer using a sequence of photolithographic steps. The component formation requires very exacting details about geometric patterns and separation between them. The process of converting the specifications of an electrical circuit into a layout is called the physical design.
Currently, the minimum geometric feature size of a component is on the order of 0.2 microns. However, it is expected that the feature size can be reduced to 0.1 micron within the next few years. This small feature size allows fabrication of as many as 4.5 million transistors or 1 million gates of logic on a 25 millimeter by 25 millimeter chip. This trend is expected to continue, with even smaller feature geometries and more circuit elements on an integrated circuit, and of course, larger die (or chip) sizes will allow far greater numbers of circuit elements.
Due to the large number of components and the exacting details required by the fabrication process, physical design is not practical without the aid of computers. As a result, most phases of physical design extensively use Computer Aided Design (CAD) tools, and many phases have already been partially or fully automated. Automation of the physical design process has increased the level of integration, reduced turn around time and enhanced chip performance.
The objective of physical design is to determine an optimal arrangement of devices in a plane or in a three dimensional space, and an efficient interconnection or routing scheme between the devices to obtain the desired functionality.
A. IC Configuration.
An exemplary integrated circuit chip is illustrated in FIG. 1 and generally designated by the reference numeral 26. The circuit 26 includes a semiconductor substrate 26A on which are formed a number of functional circuit blocks that can have different sizes and shapes. Some are relatively large, such as a central processing unit (CPU) 27, a read-only memory (ROM) 28, a clock/timing unit 29, one or more random access memories (RAM) 30 and an input/output (I/O) interface unit 31. These blocks, commonly known as macroblocks, can be considered as modules for use in various circuit designs, and are represented as standard designs in circuit libraries.
The integrated circuit 26 further comprises a large number, which can be tens of thousands, hundreds of thousands or even millions or more of small cells 32. Each cell 32 represents a single logic element, such as a gate, or several logic elements interconnected in a standardized manner to perform a specific function. Cells that consist of two or more interconnected gates or logic elements are also available as standard modules in circuit libraries.
The cells 32 and the other elements of the circuit 26 described above are interconnected or routed in accordance with the logical design of the circuit to provide the desired functionality. Although not visible in the drawing, the various elements of the circuit 26 are interconnected by electrically conductive lines or traces that are routed, for example, through vertical channels 33 and horizontal channels 34 that run between the cells 32.
B. Layout Design Process.
The input to the physical design problem is a circuit diagram, and the output is the layout of the circuit. This is accomplished in several stages including partitioning, floor planning, placement, routing and compaction.
1. Partitioning.
A chip may contain several million transistors. Layout of the entire circuit cannot be handled due to the limitation of memory space as well as the computation power available. Therefore, the layout is normally partitioned by grouping the components into blocks such as subcircuits and modules. The actual partitioning process considers many factors such as the size of the blocks, number of blocks and number of interconnections between the blocks.
The output of partitioning is a set of blocks, along with the interconnections required between blocks. The set of interconnections required is the netlist. In large circuits, the partitioning process is often hierarchical, although non-hierarchical (e.g. flat) processes can be used, and at the topmost level a circuit can have between 5 to 25 blocks. However, greater numbers of blocks are possible and contemplated. Each block is then partitioned recursively into smaller blocks.
2. Floor planning and placement.
This step is concerned with selecting good layout alternatives for each block of the entire chip, as well as between blocks and to the edges. Floor planning is a critical step as it sets up the ground work for a good layout. During placement, the blocks are exactly positioned on the chip. The goal of placement is to find a minimum area arrangement for the blocks that allows completion of interconnections between the blocks. Placement is typically done in two phases. In the first phase, an initial placement is created. In the second phase, the initial placement is evaluated and iterative improvements are made until the layout has minimum area and conforms to design specifications.
3. Routing,
The objective of the routing phase is to complete the interconnections between blocks according to the specified netlist. First, the space not occupied by blocks, which is called the routing space, is partitioned into rectangular regions called channels. The goal of a router is to complete all circuit connections using the shortest possible wire length and using only the channel.
Routing is usually done in two phases referred to as the global routing and detailed routing phases. In global routing, connections are completed between the proper blocks of the circuit disregarding the exact geometric details of each wire and terminal. For each wire, a global router finds a list of channels that are to be used as a passageway for that wire. In other words, global routing specifies the loose route of a wire through different regions of the routing space.
Global routing is followed by detailed routing which completes point-to-point connections between terminals on the blocks. Loose routing is converted into exact routing by specifying the geometric information such as width of wires and their layer assignments. Detailed routing includes the exact channel routing of wires.
In order for circuit designers to calculate the performance of ASICs, the designers need to compute the delays of the cells in the ASICs. In the present invention, two types of delays are considered. The first type of delay is the propagation delay of a cell. A propagation delay of a cell is defined as the time duration a signal takes to travel from the input to the output of a cell. The measurement point at the input is called the switching threshold. The measurement point at the output is usually the 0.5*Vdd (the power supply). A propagation delay of a cell is defined for every input to output pin combination of a cell under both the ill rising and falling input conditions. The propagation delay is also affected by a given process (P), voltage (A) and temperature (T).
The second type of delay is the setup/hold time delay which is an input constraint for sequential cells. The setup time is defined as the time duration a data signal is required to be available at the input of a cell before the clock signal transition, and the hold time is defined as the time duration a data signal is required to be stable after the clock signal transition. For the purpose of explanation, both propagation delay and setup/hold time, henceforth, will be referred as xe2x80x98delayxe2x80x99.
The following derating equation is widely used in the industry to compute the delay of a cell for a given P, V and T of a cell:
Dcase=K*Dnom where, 
Dnom=nominal delay at nominal P, V and T (e.g., P=nominal process, T=room temperature, V=supply voltage);
Dcase=delay for a given P, V and T;
K=(1+Kp)*((1+Kv (Vcasexe2x88x92Vnom))*((1+Kt(Tcasexe2x88x92Tnom));
Kp=(Dcasexe2x88x92Dnom)/Dnom;
Kv=((Dcasexe2x88x92Dnom)/Dnom)*xcex94, where xcex94=(Vcasexe2x88x92Vnom);
Kt=((Dcasexe2x88x92Dnom)/Dnom)*xcex94, where xcex94=(Tcasexe2x88x92Tnom);
The equation given above suffers from several problems when the equation is used to calculate delays. First, if Dnom equals 0 or is very small, Kp, Kv and Kt approaches infinity, thus, producing an invalid result. Second, the sign of Dnom (whether positive or negative) affects the result. For example, if the voltage supplied to a cell is reduced, the delay of the cell is suppose to increase, but if Dnom is a negative number and the above equation is used to calculate Dcase for a lower supply voltage, the calculated delay, Dcase, of the cell decreases instead of increasing, as is the case in the real world. Thus, the sign of Dnom may affect the outcome and produce an incorrect result. Third, the above equation is inaccurate because the equation is based on data sampling at a single point and uses linear curve fitting scheme to find the new delay. Fourth, the above equation is not suitable for derating setup and hold times. Finally, the above equation does not capture the dependancy of the delay on the fanout and the input ramptime (defined as the time duration an input signal takes to switch between two logic levels completely) of the cells.
It is an object of the present invention to provide methods for calculating delays for cells in an ASIC, which obviate for practical purposes the above mentioned limitations.
According to an embodiment of the present invention, the delays, including the propagation delays and the setup/hold time delays, are computed by considering not only the process (P), voltage (V), temperature (T) but also input ramptime (R) and output load or fanout (F) of the cells by fitting the delay at four corner points for derated PVT condition into a non-linear equation which is a function of P, V, T, R and F. Thus, in embodiments of the present invention, the delay characterization is a five dimensional characterization process, and this characterization space is split into (P,V,T) characterization and (R,T) characterization to reduce the characterization time and resources. The present invention provides for accurate calculation of delays for cells in ASICs.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, byway of example, various features of embodiments of the invention.